1. Field of the Invention
The present invention relates to a substrate container (pod) with a humidity removing function such as a clean box, a substrate storage box, a substrate carrying container, a substrate holding rack, or the like for storing or transporting substrates such as semiconductor wafers, photo-masks, hard-disks, etc. in a highly clean atmosphere or a highly water-free atmosphere, and a method of dehumidifying such a substrate container.
2. Description of the Related Art
When substrates such as semiconductor wafers, photo-masks, etc. are delivered or stored in the fabrication process in semiconductor fabrication factories, the yield of products tend to decrease if a small amount of particulate contaminants or gaseous impurities present in the atmosphere is attached to those substrates. Such a tendency increases as the level of circuit integration grows. With the advent of magnetic reluctance heads, magnetic disks are required to record information at an increased density, and need to be highly clean against gaseous impurities as well as particulate contaminants. Furthermore, in view of reducing the corrosion of semiconductor substrates, there has been a demand for means for keeping at a low level the humidity of an atmosphere in which those semiconductor substrates are stored.
To create a clean space for accommodating substrates such as semiconductor wafers, magnetic disks, etc. when they are delivered or stored, there have been developed clean boxes equipped with a fan motor and a HEPA (High Efficiency Particle Air) filter or an ULPA (Ultra Low Penetration Air) filter. Such clean boxes also have a filter for removing gaseous impurities, such as a chemical filter, in order to avoid the adverse effect of such gaseous impurities, and also a dehumidifying means such as a desiccant for lowering the humidity.
There is known a compact and economical dehumidifying storage device, which comprises an electrochemical element in the form of a solid polymer electrolyte film that has an anode facing inwardly of a storage container and a cathode facing outwardly of the storage container.
The chemical filter and desiccant in the clean boxes have their service life depending on the gas concentration of the environment in which they are used and the level of contamination of the stored substrates. Generally, the service life of a filter for removing gaseous impurities is shorter than the service life of a particle removing filter. The gas concentration in the air which serves as the load on the filter for removing gaseous impurities ranges from several tens to several hundreds ng/g-air, whereas the water content in the air is 5 mg/g-air at the temperature of 20xc2x0 C. It is apparent that the service life of the desiccant is shorter than the service life of the filter for removing gaseous impurities.
If there is used a dehumidifying unit capable of mechanically purging water from a circulating gas flow, rather than causing a desiccant to adsorb water in a circulating gas flow and replace the desiccant when its performance is lowered, then such a dehumidifying unit needs easy maintenance because it requires no parts replacement, and does not cause any contamination which would otherwise result from desiccant impurities.
It is known in the art that those chemical filters which adsorb gaseous impurities based on an ion exchange reaction have its impurity removing capability depending on the water content in the air, i.e., the humidity. It is necessary to keep the environment in the container or clean box at a minimum humidity level which makes it possible for the chemical filter to perform its function to remove gaseous impurities.
According to another process, the container or clean box is filled with a drying gas to achieve a low level of humidity therein. One problem with this process is that the humidity in the container rises unless the container is continuously supplied with the drying gas. The cause of the problem is that the container is made of a humidity-permeable synthetic resin and it is difficult to seal the container, which has an opening, against the molecular-level entry of material. In order to keep a low level of humidity therein in the container, therefore, it is necessary that the container be made of a humidity-impermeable material and the container have some means for removing water that tends to permeate or leak into the container.
It is therefore an object of the present invention to provide a substrate container for such as a clean box, a substrate feed container, a substrate holding rack, or the like for storing or transporting substrates, the substrate container having a dehumidifying unit which is maintenance-free and compact, is an energy saver, and is capable of controlling a level of humidity that can be achieved.
Another object of the present invention is to provide a method of dehumidifying a substrate container.
According to the present invention, there is provided a substrate container comprising a container housing having an opening for storing a substrate therethrough and a cover sealingly covering the opening, a fan motor mounted in the container housing for producing a circulating path of air in contact with the substrate in the container housing, a particle removing filter and a gaseous impurity trapping filter which are disposed in a portion of the circulating path which extends toward the substrate, and dehumidifying means for dehumidifying a space in the container housing, the dehumidifying means comprising a solid polymer electrolyte film disposed in the circulating path and voltage applying means for applying a voltage to the solid polymer electrolyte film to decompose water in air flowing in the circulating path thereby to dehumidify a space in the container housing.
The dehumidifying means further comprises an anode mounted on one surface of the solid polymer electrolyte film, and a cathode mounted on the other surface of the solid polymer electrolyte film, the voltage applying means comprising means for applying the voltage between the anode and the cathode across the solid polymer electrolyte film to control the humidity in the space in the container housing. One of the anode and cathode, or both of the anode and cathode may be provided with a catalyst attached thereto for promoting a dehumidifying action.
When a DC voltage is applied between the anode and the cathode, water in air flowing through the circulating path is decomposed to dehumidify the container housing. Specifically, when the dehumidifying means is energized, water adsorbed to the anode is decomposed into hydrogen ions and an oxygen gas by the anode catalyst. The hydrogen ions move through the solid polymer electrolyte film toward the cathode, are coupled to atmospheric oxygen on the side of the cathode, producing water, which is then discharged into the atmosphere outside of the cathode.
Since no water is accumulated in the solid polymer electrolyte film, the solid polymer electrolyte film can be used semi-permanently because its dehumidifying capability is not lowered after repeatedly used. The dehumidifying means is almost maintenance-free as the solid polymer electrolyte film does not need to be replaced. The solid polymer electrolyte film can safely be used in a highly clean environment as it does not produce contaminants. Though the solid polymer electrolyte film has a relatively small area, it has a large dehumidifying capability, and it is of a low-profile, compact structure that can easily be placed in the substrate container because it needs to be supplied with a DC electric energy that is required only to decompose the water.
It is known in the art that the ability of a filter, which operates based on an ion exchange reaction for its adsorption mechanism depends on water in air, i.e., humidity. It is necessary to keep a circulating air environment in the container housing at or higher than a minimum humidity level at which the ability of the filter can be maintained. Conventional desiccants are unable to set a desired humidity level to be reached to a desired value, and hence their ion removing capability tends to be lowered when it is combined with a dehumidifying function. The dehumidifying means according to the present invention is effective to solve the problems of the conventional desiccants. Specifically, when a certain voltage is applied across the solid polymer electrolyte film, the humidity in the container housing can be controlled at a desired level. Stated otherwise, the humidity of the air in the circulating path can be controlled depending on the voltage applied across the solid polymer electrolyte film. For example, the humidity of the air in the circulating path can be adjusted to a certain value, such as 10%, making it possible to generate an air flow with controlled humidity in the container housing. The voltage may be applied in such a pattern that when the humidity in the container housing is high immediately after the substrate container is opened, the container housing is quickly dehumidified, and then the humidity in the container housing is kept at a constant level.
Preferably, the substrate container further comprises a substrate holder for holding the substrate substantially parallel to the circulating path. The substrate thus held by the substrate holder can be supplied with clean air which has been dehumidified and from which particulate and gaseous contaminants have been removed.
Preferably, the substrate container further comprises a plurality of storage chambers for individually loading and unloading substrates or containers which hold substrates, the circulating path including air supply passage and an air discharge passage which are held in communication with the storage chambers, and common air circulating means for circulating clean air through the storage chambers through the air supply passage and the air discharge passage. The substrates in the storage chambers or the containers holding the substrates can be supplied with clean air which has been dehumidified and from which particulate and gaseous contaminants have been removed.
According to the present invention, there is also provided a method of dehumidifying a substrate container comprising the steps of producing a circulating air flow in a container housing which stores a substrate therein, and applying a voltage to a solid polymer electrolyte film placed in contact with the circulating air flow to decompose water in the circulating air flow thereby to dehumidify a space in the container housing. On a semiconductor wafer, for example, a naturally occurring oxide film grows due to water and oxygen contained in an environment in which the semiconductor wafer is stored. According to the present invention, after the semiconductor wafer is stored in a substrate transporting device, the substrate transporting device is filled with dry air to hold a water content in the substrate transporting device to a very low level for thereby suppressing the growth of the naturally occurring oxide film. At the same time, particulate contaminants and organic materials are effectively prevented from being adsorbed to the semiconductor wafer as the ability of the solid polymer electrolyte film is not lowered even when the low-humidity air is supplied.
Further, when dehumidifying the container atmosphere by utilizing the dehumidifier with its capability to full extent, the dehumidifying unit and a moisture absorbing material are both installed in the container, it is then possible to lower the humidity rapidly by operating a fan and other similar means to facilitate the container gases to contact the dehumidifying unit and the moisture absorber. Moisture absorbing material may include most preferably such materials as activated charcoal, activated carbon fiber, ion exchange resins, ion exchange non-woven cloths, fabrics or fibers, silica-gel and zeolite to adsorb gaseous impurities, and they should be dried prior to use so as to drive out moistures retained in the material. Of the ion exchange materials, those containing sulfonic acid are excellent moisture absorbers. By pre-treating to remove moisture beforehand, they can act as a superior desiccant. Also, if the ambient condition is about 15% RH, basic gases can also be absorbed effectively. In actual operation, it is most effective to maintain the moisture absorbing material in a dry condition in the dehumidifying unit constantly so as to preserve the initial state that exhibits the most moisture absorbing capacity.
If the container housing is filled with a drying gas when the solid polymer electrolyte film is used to dehumidify the container housing, then the dehumidifying rate can be increased, and the container housing can be maintained at a low level of humidity after the supply of the drying gas is stopped. An electric current consumed by the solid polymer electrolyte film is proportional to the humidity of the environment to be dehumidified. Consequently, as the humidity of the environment to be dehumidified is lower, the electric current consumed by the solid polymer electrolyte film is lower. Accordingly, supplying the drying gas when the solid polymer electrolyte film is used to dehumidify the container housing is effective to minimize the current consumption. This is particularly advantageous when a secondary battery with a limited battery capacity is used to energize the solid polymer electrolyte film.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, which illustrate preferred embodiments of the present invention by way of example.